In recent years, sensing of temperature or the like by an optical signal has begun to be performed in a high-temperature environment.
In optical fibers used in such an environment, polyimide resins having high heat resistance have been being used as an optical fiber coating material.
In general, as a method for coating a glass fiber with a polyimide resin, there is known a method in which a varnish of a polyimide resin or a precursor thereof dissolved in a solvent such as NMP is used, this polyimide varnish is applied on the glass fiber surface, and the polyimide resin is then crosslinked for curing by heating in a crosslinking furnace. However, such a method involves such a problem that the solvent used in the varnish must be volatilized, so that the productivity is low (up to about 10 m/min).
In addition, a transmission loss of the optical fiber is significantly affected by physical properties or structure of the resin to be coated around the periphery of the glass fiber. Even if the coating resin is in an already stabilized state after curing, a physical minute residual stress or residual strain may be generated in the coating resin due to a stress or strain newly loaded on the optical fiber in a process of rewinding, etc. In the case where the distribution of this residual stress/strain is non-uniform in the optical fiber, an excessive transmission loss, called a microbending loss may be generated. Accordingly, the optical fiber is also similarly required to have a structure in which a microbending loss is suppressed.
Patent Document 1 discloses an energy (ultraviolet ray, etc.)-curable silicone resin (silicon compound) having a cyclosiloxane structure having an epoxy group introduced thereinto and a linear siloxane structure and describes that a cured product having excellent curing properties, heat resistance and flexibility is obtained by using this energy-curable silicone resin jointly with an epoxy-curable compound (curing agent).